1. Voltage, Current, and Resistance
DC/AC Fundamentals: A Systems Approach
Thomas L. Floyd
David M. Buchla
Voltage, Current, and Resistance
Chapter 2

2. The Bohr atom is useful for visualizing atomic structure.
Ch.2 Summary
The Bohr Atom
The Bohr atom is useful for visualizing atomic structure.
The nucleus is positively charged and has the protons and neutrons.
Electrons are negatively charged and in discrete shells.
The atomic number is the number of protons and determines the particular element.
In the neutral atom, the number of electrons is equal to the number of protons.

3. The Valence Shell Semiconductor Ch.2 Summary
The outer shell is called the valence shell. Electrons in this shell are involved in chemical reactions and they account for electrical and thermal conductivity in metals.
A neutral silicon (Si) atom is shown. There are four electrons in the valence shell.
Is Si a conductor, insulator, or semiconductor?
Semiconductor

4. The Valence Shell Ch.2 Summary
Metals have one, two or three electrons in the valence shell. The atom illustrated here is a sodium (Na) atom with only one electron in its outer shell.
Sodium is highly reactive, and easily gives up its single valence electron. For this reason, it is not used in electrical work.
Non-metals have either complete or nearly compete outer shells, so they make poor electrical conductors.
Sodium atom

5. Electrical Charge Ch.2 Summary
There is a force (F) between electrical charges. Like charges repel; unlike charges attract.
The force is directly proportional to charge.
The force is inversely proportional to square of distance.

6. Voltage e- e- e- e- e- e- e- e- Ch.2 Summary
Force is required to move a charge against an electric field. e- e-
When force is applied over a distance, work is done. Work done in moving a charge against the electric field leads to the definition of voltage: e- e- e- e- e-
Voltage is the work per charge done against the electric field. e-

7. The defining equation for voltage is
Ch.2 Summary
Voltage
The defining equation for voltage is
One volt is the potential difference (voltage) between two points that uses one joule of energy to move one coulomb of charge from one point to the other.

8. Voltage is responsible for establishing current.
Ch.2 Summary
Voltage
Voltage is responsible for establishing current.
This is an example of a single cell battery.
Sources of voltage include batteries, solar cells, fuel cells, and generators. A Cu-Zn battery, such as you might construct in a chemistry class, is shown.

9. Ch.2 Summary
Batteries
An automobile battery is an example of a multiple cell battery. Like all batteries, the automotive battery does not store charge – it stores chemical energy that can be used to generate current when an external path is provided to allow its chemical reaction to proceed.
Rather than saying “charging” a battery, it is more accurate to say “reversing the chemical reaction” in a battery.

10. Ch.2 Summary
Fuel Cells
A fuel cell converts chemical energy into dc voltage by combining a fuel (usually hydrogen) with an oxidizing agent (usually oxygen).
Hydrogen and oxygen react to form water.
This process differs from batteries in that the reactants constantly flow into the cell where they combine and produce electricity.

11. Ch.2 Summary
Voltage
Ideally, a voltage source can provide a constant voltage for any current required by a circuit. I V
The IV curve for an ideal voltage source has a constant voltage for all current.
In practice, ideal sources do not exist, but they can be closely approximated by actual sources.

12. Ch.2 Summary
Current
Current (I) is the amount of charge (Q) that flows past a point per unit of time (t). The defining equation is:
One ampere is a number of electrons having a total charge of 1 C moving through a given cross section per second (s).
What is the current if 2 C passes a point every 5 s?
0.4 A

13. Ch.2 Summary
Current
Ideally, a current source can provide a constant current for any load. I V
The IV curve for an ideal current source has a constant current as indicated by the straight line.

14. Current Sources Ch.2 Summary
Current sources are not as common as voltage sources, but they are useful for production testing.
The units shown here include current sources and built-in measurement instruments, and can operate using a built-in microprocessor to direct a test sequence.
Courtesy of Keithley Instruments

15. Resistance (R) is the opposition to current.
Ch.2 Summary
Resistance
Resistance (R) is the opposition to current.
One ohm (1 W) is the resistance if one ampere (1 A) is in a material when one volt (1 V) is applied.
Conductance (G) is the reciprocal of resistance.
Components designed to have a specific amounts of resistance are called resistors.

16. Resistance Color Code Ch.2 Summary
Resistance value, first three bands:
First band – 1st digit
Second band – 2nd digit
Third band – Multiplier *
(number of zeros following second digit)
Fourth band - tolerance
* For resistance values less than 10 W, the third band is either gold or silver. Gold is for a multiplier of 0.1 and silver is for a multiplier of 0.01.

17. Ch.2 Summary
Question
What is the resistance and tolerance of each of the four-band resistors?
5.1 kW ± 5%
820 kW ± 10%
47 W ± 10%
1.0 W ± 5%

18. Alphanumeric Labeling
Ch.2 Summary
Alphanumeric Labeling
Two or three digits, and one of the letters R, K, or M are used to identify a resistance value.
The letter is used to indicate the multiplier, and its position is used to indicate decimal point position.

19. Variable Resistors Ch.2 Summary
Variable resistors include the potentiometer and rheostat. The center terminal of a variable resistor is connected to the wiper.
To connect a potentiometer as a rheostat, one of the outside terminals is connected to the wiper.

20. Wire Resistance Ch.2 Summary 6.46 W 6.46 W
Sometimes, the resistance of wires must be accounted for. The equation for wire resistance is:
where  = resistivity in CM-W/ft
l = length in feet
A = cross sectional area in circular
mils (CM)
What is the resistance of 400 feet of 22 gage copper wire? The area is 642 CM and the resistivity of copper is CM-W/ft. The table value for resistance/1000 feet of 22 gage wire is W/1000 feet.
The resistance of 400 feet is 0.4  W =
6.46 W
By the equation,
6.46 W

21. A basic electric circuit consists of
Ch.2 Summary
The Electric Circuit
A basic electric circuit consists of
a voltage source
a path
a load
An example of a basic circuit is a flashlight, which has each of these plus a control element – the switch.

22. The Electric Circuit Ch.2 Summary
Circuits are represented pictorially with schematics.
For example, the flashlight can be represented as shown below.
Battery (2 cells)
Switch
Lamp

23. Ch.2 Summary
Switches
Switches are commonly used to control circuits by either mechanical or electronic means.
The pole refers to the movable arm of a switch.
The throw refers to the number of contacts that are affected by a single switch action.
SPST
SPDT
DPST
DPDT

24. Ch.2 Summary
The DMM VW
The digital multimeter, or DMM, is an important multipurpose instrument which can measure voltage, current, and resistance.
Many include other measurement options.

25. Analog Meters Ch.2 Summary
An analog multimeter is also called a VOM (volt-ohm-milliammeter). Analog meters measure voltage, current, and resistance. The user must choose the range and read the proper scale.

26. Selected Key Terms Ch.2 Summary Ampere The unit of electrical current.
AWG
Charge
Circuit
The unit of electrical current.
(American Wire Gauge) A standardization based on wire diameter.
An electrical property of matter that exists because of an excess or a deficiency of electrons. Charge can be either + or -.
An interconnection of electronic components designed to produce a desired result. A basic circuit consists of a source, a load, and an interconnecting path.

27. Selected Key Terms Ch.2 Summary Conductance
Coulomb
Current
Electron
Ground
Ohm (W)
The ability of a circuit to allow current. The unit of conductance is the sieman (S).
The unit of electrical charge.
The rate of flow of electrical charge.
A basic particle of electrical charge in matter. The electron possesses a negative charge.
The common or reference point in a circuit.
The unit of resistance.

28. Selected Key Terms Ch.2 Summary A three-terminal variable resistor.
Potentiometer
Resistance
Rheostat
Siemens
Volt
Voltage
The opposition to current. The unit is the ohm (W).
A two-terminal variable resistor.
The unit of conductance.
The unit of voltage or electromotive force.
The amount of energy per charge available to move electrons from one point to another in an electric circuit.

29. Quiz Ch.2 Summary 1. The atomic number of an element is the number of
a. protons in the nucleus
b. neutrons in the nucleus
c. protons plus neutrons in the nucleus
d. electrons in the outer shell

30. Quiz Ch.2 Summary 2. Valence electrons are a. in the outer shell
b. involved in chemical reactions
c. relatively loosely bound
d. all of the above

31. Ch.2 Summary
Quiz
3. The atomic particle responsible for electrical current in solid metallic conductors is the
a. proton
b. electron
c. neutron
d. all of the above

32. Ch.2 Summary
Quiz
4. The symbol for charge is
a. C
b. W
c. Q
d. W

33. Ch.2 Summary
Quiz
5. The definition for voltage is a. b. c. d.

34. Quiz Ch.2 Summary 6. A battery stores a. electrons b. protons c. ions
d. chemical energy

35. Quiz Ch.2 Summary 7. The unit of conductance is the a. ohm b. coulomb
c. sieman
d. ampere

36. Ch.2 Summary
Quiz
8. A four-color resistor with the color bands gray-red-black-gold is
a. 73 W
b. 82 W
c. 680 W
d. 820 W

37. Quiz Ch.2 Summary 9. The color bands for a 330 kW ± 5% resistor are
a. red-red-brown-gold
b. orange-orange-yellow-gold
c. yellow-yellow-red-gold
d. yellow-yellow-green-gold

38. Quiz Ch.2 Summary 10. The circular mil is a unit of a. length b. area
c. volume
d. resistance

39. Answers Ch.2 Summary 1. a 2. d 6. d 3. b 7. c 4. c 8. b 5. c 9. b